1. Field of the Invention
The present invention relates in general to a method for reducing a printing position error. More specifically, the present invention relates to a method for reducing a printing position error by setting an optimal initial acceleration for a motor used in feeding printing papers.
2. Description of the Related Art
In a thermal imaging printing system, which performs the printing operation on both sides (or surfaces) of a printing paper by applying heat using a thermal printhead (TPH), an encoder wheel for feeding the printing paper repeatedly rotates in the forward and backward directions.
In order to obtain high quality prints, it is important to accurately predict a feeding distance of the printing paper along the forward and backward rotation of the encoder wheel. That is, in heating both sides of the paper by the TPH, an accurate, clear output image can be obtained when printing start positions on both sides coincide with each other.
FIG. 1 is a diagram of a conventional TPH printing system.
As shown in FIG. 1, the TPH printing system according to one embodiment of the related art comprises a sensing unit 10, an encoder wheel 20, a feeding roller 30, a pressing roller 40, a printing paper 50, and a TPH 60.
The sensing unit 10 transmits a printing paper detection signal to a driving control unit (not shown) as the printing paper 50 is fed, and the encoder wheel 20 rotates in the forward or backward direction under the control of the driving control unit.
The feeding roller 30 is rotatably mounted on a shaft of a drive motor (not shown) that is controlled by the driving control unit. Therefore, the feeding roller 30 rotates in the forward or backward direction along the rotation of the drive motor, and feeds the printing paper 50 in the forward or backward direction.
When the printing paper 50 is fed by the feeding roller 30 and the pressing roller 40, the TPH 60 applies heat to both sides of the printing paper 50 in order to print a target image.
The printing operation in the TPH printing system of the related art always accompanies the paper feeding in the forward and backward directions. To do so, the drive motor connected to the driving control unit rotates the feeding roller 30 and the encoder wheel 20 coupled thereto, and the printing paper 50 is transported by the rotation of the feeding roller 30 and the encoder wheel 20.
However, at the start of the rotation, the printing paper 50 resists the paper feed force in the horizontal direction generated by an initial acceleration of the drive motor, and therefore, the printing paper 50 moves slightly laterally in the rotation direction of a driving shaft commonly coupled to the feeding roller 30 and the encoder wheel 20.
However, since this slight movement of the printing paper 50 is not reflected in the rotation of the feeding roller 30 and the encoder wheel 20, the feeding distance of the printing paper 50 is not included in the number of counts of the encoder wheel 20.
Therefore, there is a small difference between the actual distance that the printing paper 50 is conveyed and the measured feeding distance of the printing paper 50 determined by the number of counts of the encoder wheel 20. This difference exists in both forward rotation and backward rotation of the encoder wheel 20, and is influenced by the initial acceleration set for the drive motor.
Further, even though the actual print quality generated by the TPH printing system becomes deteriorated, the same initial acceleration is applied to paper feeding, without considering the slight horizontal shift of the driving shaft itself. As a consequence, the deterioration in the print quality of an image can not be prevented.
Accordingly, a need exists for a system and method for maintaining or improving print quality by eliminating undesired paper movement at the start of feeding roller rotation.